Recently the change from expensive electrotinplates to cheaper tin free steel (TFS-CT) having double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide as well as a decrease in the weight of the tin coating in electrotinplates has rapidly taken place in the field of food cans.
This is because the tin used for the production of tinplate is very expensive and there is concern over the exhaustion of tin resources.
An ordinary metal can consists of two can ends and a single can body, except for drawn cans. In the case of tinplate, the scaming of the can body is generally carried out by soldering. In this soldering process, however, it is impossible to decrease the weight of tin coating on the tinplate to under 2.8 g/m.sup.2, because it is difficult to stabilize the soldering process when the weight of the tin coating is under 2.8 g/m.sup.2. From the regulation of lead content in the solder used for the seaming of the tinplate can body in the field of food cans, the seaming of the tinplate can body is widely carried out by electric welding.
A lap seam welding, for instance, the Soudronic process, has been recently used for the seaming of the tinplate can body.
In this process, it is desirable to decrease the tin coating weight in the tinplate, but the weldability of tinplate becomes poor with a decrease of the tin coating weight.
On the other hand, the seaming of a TFS-CT can body is generally carried out with nylon adhesives by using the Toyo Seam (Trade name) and Mira Seam (Trade name) method.
Another method of seaming a TFS-CT can body by electric welding is also well known. In the case of the seaming of a TFS-CT can body by electronic welding, however, the metallic chromium layer and the hydrated chromium oxide layer must be mechanically or chemically removed from the TFS-CT surface in order to easily weld the TFS-CT can body at high speed. Therefore the corrosion resistance in the welded part of the TFS-CT can body becomes remarkably poor, even if this welded part is coated with lacquer after welding.
From the background described above, the development of a can material which is cheaper than tinplate and is easily welded at high speed without the removal of the plated layer, has been required in the field of food cans.
Recently, various surface treated steel sheets have been proposed as a can material which can be easily welded at high speed without the removal of the plated layer. For instance, the following surface treated steel sheets have been proposed: (a) Lightly tin coated steel sheet (LTS) with below about 1.0 g/m.sup.2 of tin which is reflowed or unreflowed after tin plating (Japanese Patent Publication Nos. Sho 56-3440, Sho 56-54070, Sho 57-55800, and Laid-Open Japanese Patent Application Nos. Sho 56-75589, Sho 56-130487, Sho 56-156788, Sho 57-101694, Sho 57-185997, Sho 57-192294, Sho 57-192295 and Sho 55-69297). (b) Nickel preplated LTS with below about 1.0 g/m.sup.2 of tin (Laid-Open Japanese Patent Application Nos. Sho 57-23091, Sho 57-67196, Sho 57-110685, Sho 57-177991, Sho 57-200592 and Sho 57-203797 ). (c) Nickel plated steel sheet with chromate film or phosphate film (Laid-Open Japanese Patent Application Nos. Sho 56-116885, Sho 56-169788, Sho 57-2892, Sho 57-2895, Sho 57-2896, Sho 57-2897, Sho 57-35697 and Sho 57-35698). (d) TFS-CT having double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide which is obtained by some special methods such as cold rolling after TFS treatment (Laid-Open Japanese Patent Application No. Sho 55-48406), porous chromium plating (Laid-Open Japanese Patent Application No. Sho 55-31124) and a cathodic treatment of a steel sheet in chromic acid electrolyte with fluoride but without anions such as sulfate, nitrate and chloride ion (Laid-Open Japanese Patent Application No. Sho 55-18542).
However, LTS and nickel preplated LTS above-identified as (a) and (b) are slightly more expensive than TFS-CT.
Furthermore, these have not only a narrower available current range for sound welding than that in tinplate, but also poor lacquer adhesion compared with that in TFS-CT, although these can be welded without the removal of the plated layer. The reason why the available current range for sound welding in LTS and nickel preplated LTS is narrower than that in tinplate is considered to be that the amount of free tin in these is smaller than that in tinplate and also further decreases because of changes of free tin to iron-tin alloy by heating for lacquer curing. Nickel plated steel sheet with chromate film or phosphate film above-identified as (c) also has a narrower available current range for sound welding than that of LTS or nickel preplated LTS. Furthermore, the corrosion resistance of nickel plated steel sheet is poorer than that of TFS-CT, although the lacquer adhesion of nickel plated steel sheet is good. Particularly, pitting corrosion in the defective part of the lacquered nickel plated steel sheet may occur easily from acidic foods such as tomato juice because the electrochemical potential of nickel is more noble than that of the steel base and metallic chromium.
It is considered that the welding of TFS-CT shown above in (d) without the removal of the TFS-CT film at high speed is very difficult because the oxide films having high electric resistance are formed by the oxidation of metallic chromium and exposed steel base through the plating pores by the dehydration of hydrated chromium oxide during heating for curing the lacquer coated on the TFS-CT can body, although TFS-CT shown in (d) may be welded when it is not heated before welding.
As described above, various surface treated steel sheets proposed in (a), (b), (c) and (d) have various problems in their production cost and their characteristics as a can material which can be easily welded without the removal of the plated layer at high speed.
Accordingly, it is the first objective of the present invention to provide a surface treated steel sheet having excellent weldability, that is, easily being welded without the removal of the plated layer at high speed, and having excellent corrosion resistance after lacquering such as in a TFS-CT.
It is the second objective of the present invention to provide a method for the continuous production of a surface treated steel sheet having an excellent weldability.